1. Field of the Invention
The present invention relates to a battery electrode having a plaque made of a sponge-like porous metal matrix having a multiplicity of cells therein connected with each other three-dimensionally.
2. Description of the Prior Art
Conventional electrodes for the primary battery presently on the market are produced by a method in which a powder mixture mainly comprising an active material is impregnated directly in the battery case or molded with a support such as a grating, a screen, a punched metal or an expanded metal arranged at the central part of the mixture. This production method is simple and has the advantage that a number of active materials are capable of being impregnated, thus making it suitable for the primary battery which does not require a very high strength. Representative secondary batteries include a lead acid battery and a nickel-cadmium storage battery. The electrodes for the lead acid battery are made by a method in which a paste mainly comprising at least an active material is coated with a lead grating or an expanded metal as a support or a method in which a cylinder of metal having numerous minor pores is impregnated with an active material. These methods provide a simple production process since about the only main work involved is the coating of an active material or the impregnation thereof in the cylinder directly. The electrodes of the nickel-cadmium storage battery, on the other hand, are made by a method in which an active material is filled in a sintered plaque of nickel powder or a method in which an active material is directly filled in a metal bag having a multiplicity of small pores. The former method was developed after the World War II and requires a rather complicated process of immersing the electrodes in a salt of the active material to convert the salt into an active material. In spite of this, the electrode made by this method is superior to the electrodes made by the other methods in strength and electrical performances. The latter method is used for making a pocket-plate electrode and simple in view of the fact that the active material powder is filled directly, although the electrical performances of the electrode thus made are inferior to those of the electrode using a sintered plaque.
The conventional methods for producing a battery and electrodes thereof are roughly described above. At the present, earnest efforts are being made to develop a secondary battery, or especially an alkali storage battery (mainly a nickel-cadmium storage battery) having a non-sintered electrode of other than pocket-plate having electrical performances substantially identical to those of a sintered electrode superior in strength and which can be impregnated with an active material at high density by a simple production process. Such methods are disclosed by U.S. Pat. No. 2,474,502 and No. 2,694,743 for a sponge-like porous metal matrix and by U.S. Pat. No. 3,287,164 and No. 3,597,829 for an electrode using such a sponge-like porous metal matrix. This sponge-like porous metal matrix is made by the process as mentioned below.
In one method, a sponge of resin is plated with a metal and after removing the resin, the metal is annealed. According to a second method, a metal powder is filled in a resin sponge and the whole structure is sintered. According to still another method, a mixture of a material acting as a pore-promoting agent and a metal powder is sintered or molten, and any pore-promoting agent that may remain is removed by etching or the like. In a further method, a gas is blown into a molten metal and cooled with bubbles kept therein followed by application of a high pressure thereby to remove the films between the bubbles. The method in which the resin is plated already finds an industrial application. In any way, the sponge-like porous metal matrix has pores which are generally larger in average diameter than the pores of a sintered plaque of a metal powder (about 1 .mu.m). This average pore diameter is suitable for the grain size of the active material and makes it possible to directly fill it. The sintered substrate, by contrast, has a smaller average pore diameter, so that it is almost impossible to fill the powder directly. Instead, after the plaque is immersed in a salt solution of the active material, it is necessary to convert the salt impregnated in the plaque into an active material, or the plaque must be cathodically polarized in a salt solution to get the active material. This is indicative of the fact that in the case of a sponge-like porous metal matrix, the active material is capable of being filled in the substantially same simple production process as that for a non-sintered electrode. Another feature of the electrode using a plaque of sponge-like porous metal matrix resides in that in view of the fact that the active material is completely covered by a metal grating, a long service life cycle and a high electron conductivity of the electrode results, thus making it suitable for high-rate discharge. Further, the porosity is easily capable of being increased beyond that of the sintered plaque (about 80%), thus increasing the density of the filled active material. In the case of a sponge-like porous metal matrix made by plating, for instance, the porosity of up to a maximum of approximately 98% is possible. In continuous production in industrial applications, however, the porosity of about 96% or less is desirable to maintain the strength of a plaque 1 to 2 mm thick. The sponge-like porous metal matrix which has a porosity about 20% higher than that of the sintered plaque (about 80%) is capable of being filled with a proportionately greater amount of the active material.
The electrode having a plaque made of a sponge-like porous metal matrix mentioned above is superior in various electrical performances as a battery electrode and is capable of being impregnated with an active material of a large volume by a simple process. Therefore, it is suitable for the primary battery requiring a high-rate discharge performance as well as for the secondary battery. It may also be used as a spiral electrode.
As a result of an application of an electrode having a plaque of the sponge-like porous metal matrix having the features mentioned above to a nickel electrode, the advantages mentioned below have been found.
By plating a resin sponge with nickel, a sponge-like porous metal matrix of substantially uniform grating diameters (average porosity of 95% and number of cells 55 per inch) is obtained. This is used as a plaque which is filled with a paste of a mixture powder mainly comprising hydroxide nickel powder (86% by weight), resulting in the filling density of 300 to 320 mAh/cm.sup.3. When pressure of 400 kg/cm.sup.2 is applied, the density is improved to 430 to 450 mAh/cm.sup.3. Although this is higher than the density of 350 to 400 mAh/cm.sup.3 for an ordinary sintered plaque, it has been found that if the sectional area of the grating near the surface of the plaque is increased and that at the central part thereof is decreased by changing the plating conditions such as the current density or agitation for making the sponge-like porous metal matrix, a greaater amount of active material is capable of being impregnated regardless of the same porosity of the whole structure and this high density is maintained after pressure work. A sectional view of this sponge-like porous metal matrix is shown schematically in FIG. 2. Specifically, a foamed resin is coated with carbon black uniformly inside and plated on both sides with nickel with substantially no agitation at the current density of 3.0 A/cm.sup.2 in Watt solution for five minutes. The resulting structure is washed in water and the resin is removed by roasting, followed by an annealing process in a hydrogen environment at 800.degree. C. for 30 minutes, thus producing a sponge-like nickel porous metal matrix having the average porosity of 96%, the sectional area of the grating near the surface of about 2800 .mu.m.sup.2, that at the central part of about 700 .mu.m.sup.2, and the number of cells of 55 per inch. This porous sheet is pressurized between rollers, so that the surface thereof is flattened and the porosity thereof is controlled at 95%. This is used as a plaque which is filled with a paste of a powder mixture containing 86% by weight of hydroxide nickel of average grain size of 25 to 150 .mu.m. This structure is dried and subjected to pressure between flat plates at 400 kg/cm.sup.2. Thus, the density of 500 to 520 mAh/cm.sup.3 of the active material in the electrode is obtained.
In the case where a similar foamed resin is plated with nickel on both sides thereof at the current density of 1.0 A/cm.sup.2 while sufficiently agitating and the resulting sponge-like nickel porous metal matrix having a uniform sectional area of gratings is used as a plaque which is filled with a similar paste mainly comprising a hydroxide nickel powder and subjected to a pressure of 400 kg/cm.sup.2, on the other hand, the density of the filled active material of the electrode is 430 to 450 mAh/cm.sup.3.
Before pressure work, the density in the latter case is 300 to 320 mAh/cm.sup.3 which compares to the density of 350 to 380 mAh/cm.sup.3 in the former case. This shows that more active materials are used with the plaque in the former case than in the latter case. It has also been found that the sectional area of the gratings at the central part of the resin sponge relative to the sectional area of the gratings near the surface thereof tends to decrease with the increase in current density or with the decrease in agitation for palting or with the increase in the thickness of the resin sponge.